 <nav class="navbar navbar-expand-lg navbar-light bg-light mb-2">
      <a class="navbar-brand" href="bootstrap.php">留言板</a>
      <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav"
        aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
      </button>
      <div class="collapse navbar-collapse" id="navbarNav">
        <ul class="navbar-nav ml-auto">
    <?php if($user_id < 1):?>
          <li class="nav-item active">
            <a class="nav-link" href="reister.php">注册</a>
          </li>
          <li class="nav-item active">
            <a class="nav-link" href="login.php">登录</a>
          </li>
    <?php else:?>
          <li class="nav-item active">
            <a class="nav-link" href="avatar.php">头像</a>
          </li>
          <li class="nav-item active">
            <a class="nav-link" href="password.php">密码</a>
          </li>
          <li class="nav-item active">
            <a class="nav-link" href="logout.php">退出</a>
          </li>
    <?php endif;?>
        </ul>
      </div>
    </nav>